Aqueous processing offers a convenient route to the preparation of nanoparticle dispersions, such as aqueous dispersions of cerium dioxide nanoparticles. However, to be useful in applications such as a fuel-borne catalyst, particles such as cerium dioxide nanoparticles must exhibit stability in a non-polar medium, for example, diesel fuel, such that these particles remain suspended in the fuel and do not settle out. Thus, these particles, although most readily formed and suspended in a highly polar aqueous phase, must then be transferred to a substantially non-polar phase. This problem is conventionally addressed by the use of particle stabilizers. However, there is need for additional improvement as most particle stabilizers used to prevent particle agglomeration in an aqueous environment are ill-suited to the task of stabilization in a non-polar environment. When placed in a non-polar solvent, such particles tend to immediately agglomerate and, consequently, lose some, if not all, of their desirable particulate properties. Thus, it would be desirable to form stable nanoparticles in an aqueous environment, retain the same stabilizer on the particle surface, and then be able to transfer these particles to a non-polar solvent, wherein the particles would remain stable and form a homogeneous mixture or dispersion. Availability of a simplified and economical transfer process would, for example, eliminate the necessity for changing the affinity of a surface stabilizer from polar to non-polar. Changing stabilizers can involve a difficult displacement reaction or separate, tedious isolation and re-dispersal methods such as, for example, precipitation and subsequent re-dispersal with a new stabilizer using, for instance, a ball milling process, which can take several days.
While a less polar water-miscible solvent may be combined with an aqueous particle dispersion, it is often necessary to remove water from the resulting mixture in order to achieve the desired solvent polarity reduction and to increase particle suspension density (i.e. concentration). In general, the process of altering the ratio of components in a multi-component solvent mixture is referred to as solvent shifting. Effective means for removing salts and adding water via diffusion through semi-permeable (semi-porous) membranes are well known in the filtration art in the form of dialysis procedures. In dialysis, an aqueous solution or particle dispersion to be purified is placed into a dialysis bag (internal phase), and typically suspended in an aqueous (external phase) bath, from which water diffuses into the bag while salts diffuse out through holes in the semi-permeable dialysis membrane, driven only by concentration gradients (osmosis). The external water bath is changed periodically to restore the concentration gradients that are the source of the osmotic pressure.
Dialysis and diafiltration methods have been employed to purify biological materials by replacing minor amounts of organic solvents, organic surfactants, reaction by-products and salts, with water in order to reduce the toxicity of the final material. Diafiltration, sometimes referred to as cross-flow microfiltration, is a transverse flow filtration method that typically employs a bulk aqueous solvent flow transverse to a semi-permeable membrane. Using this technique, water and dissolved salts under pressure diffuse in a direction tangential to the bulk flow and pass through holes in the semi-permeable membrane. Water is typically added back into the feed-stream or sample reservoir to maintain volume. Diafiltration is commonly employed to purify aqueous protein solutions, for example. The pore sizes of semi-permeable membranes used in diafiltration columns are typically characterized by the molecular weight cut-off (MWCO) value. In practice, the column will retain about 90% of dissolved proteins of a molecular weight greater than the MWCO. Diafiltration columns are typically constructed of materials that are compatible with aqueous solvent (e.g. polyurethane internal glue, polycarbonate and polysulfone casings). Once more, filtration methods employing semi-permeable membranes, such as dialysis and diafiltration, typically result in a net addition of water such that a solvent shift to increased polarity is achieved.
Conventional diafiltration techniques have been used to help purify an aqueous-based, polar dispersion of nanoparticles, purifying and maintaining a highly polar continuous phase. For example, in regard to purification of nanoparticles by dialysis and diafiltration, Limayen et al., Separation and Purification Technology 38 (2004)1-9, describe the removal of organic (ethyl acetate) solvent and polyvinyl alcohol surfactant from an aqueous suspension of drug (indomethacin) loaded nanocapsules by cross-flow microfiltration, wherein pure water is added to the feed-stream during the final continuous diafiltration (constant volume) step. Dalwadi et al., Pharmaceutical Research 22 (2005) 2154-2162, studied the removal of an organic surfactant (polyvinyl alcohol) from an aqueous dispersion of poly(lactide-co-glycolide) nanoparticles by a variety of methods, including (1) a dialysis technique using freshwater as the external phase, and (2) a diafiltration technique in which the feed was diluted with water at the same rate as filtrate was generated. Feeney et al., J. Am. Chem. Soc. (2006) 128, 3190-3197, studied the purification and size-separation of water-soluble thiol-stabilized 3-nm gold nanoparticles, concluding that diafiltration is rapid and superior to other techniques, including dialysis, a combination of solvent washes, chromatography, and ultracentrifugation, in removing residual thiol ligands and disulfides. Water solvent was added to the retentate reservoir to maintain a constant volume (continuous) diafiltration process. In each of these studies teaching purification of nanoparticle dispersions, diafiltration is conducted in a continuous mode with addition of pure water to the feed-stream or sample/retentate reservoir, removing non-polar organic materials or other less polar sobstances to provide a purified polar, aqueous-based dispersion. None of these references employs dialysis or diafiltration for solvent shifting in the opposite direction, to effect a net removal of water or other polar component and to substitute a solvent of reduced polarity.
Thus, there remains a need for an efficient and economical method to synthesize stable nanoparticles, such as cerium dioxide nanoparticles and transition metal-containing cerium dioxide nanoparticles, in a polar, typically aqueous environment, and then transfer these particles to a less polar or, ultimately, a non-polar medium, wherein a stable homogeneous dispersion is maintained.